The note, written by the assistant secretary of the Department of Justice, alerted Ms Fitzgerald “a serious criminal complaint”, which had always been denied by Sgt McCabe, was raised at the commission.

The email was sent to a number of people including the then Justice Minister. During leaders’ questions, Fianna Fail TD Jim O’Callaghan said he had a “strong suspicion” efforts were being made to keep the email secret.

But Ms Fitzgerald hit back: “There were no efforts to suppress that email by me. There were no efforts to suppress it.

“I am speaking for myself. I don’t believe there were by other people either.

“There was a process being followed in the department in relation to the email.”

During the robust exchanges, the Tanaiste also claimed to have “continuously” acted on behalf of whistleblowers.

She added: “In terms of the email, what I have said is accurate about when I was told about it. That is absolutely accurate.

“To say I did not act is not correct. I did not interfere with the legal strategy.

Fianna Fail's Jim O'Callaghan TD (Image: Gareth Chaney Collins)

“And I am really surprised by some of the suggestions that have been made on the floor of the House by Deputy (Jim) O’Callaghan who says I should have discussed the legal strategy and so on.

“That would have been illegal.

“It would have been a criminal act for me to try and influence the legal strategy of the Garda Siochana.”

“You are sticking to your script and I for one am not taken for a moment by the bluster and the long enunciation of a list of your virtues in office.

“The fact is when it counted and when it mattered and when this email arrived in your inbox and when you became aware of a malicious strategy to malign this man on the basis of charges that had been dismissed and disproven you failed to act.”

Ms Fitzgerald said on Tuesday she had been aware a “serious criminal” complaint was levelled at Sgt McCabe during the O’Higgins inquiry. She has also insisted she did not know about a smear campaign against the officer until 2016.

Sinn Fein Mary Lou McDonald TD (Image: Gareth Chaney Collins)

Last year it emerged the legal team appointed by the former Garda Commissioner was instructed to question Sgt McCabe’s motivation and credibility at the O’Higgins Commission, which was set up in February 2015 to examine allegations of Garda malpractice.

The strategy pursued against Sgt McCabe is one of a number of issues being examined by the disclosures tribunal.

In a statement released after question time, Sinn Fein justice spokesman Donnchadh O Laoghaire said: “This is on the back of her inability to satisfy queries about her lack of action when she first became aware of the strategy devised by the Garda Commissioner’s legal team in an attempt to smear the name of whistleblower Sgt Maurice McCabe.

“We believe we have at this stage given the Tanaiste ample opportunity to be fully transparent on all outstanding questions and it is regrettable she has not taken it.”